1. Field of the Invention
The present invention relates to generally an air-fuel intake system for an internal combustion engine and more particularly to an improvement of a throttling system thereof.
2. Description of the Prior Art
The conventional intake systems comprise in general a fuel supply means such as a carburetor and a fuel injection valve, an intake pipe, a butterfly type throttle valve disposed within the intake pipe, and an intake manifold for distributing the air-fuel mixture to the engine cylinders.
The intake systems of the type described above has a problem that the uniformity of air-fuel mixture distribution cannot be completely ensured so that the high and reliable engine performance cannot be ensured and the emission of pollutants is high. The main cause of this problem is that the fuel supplied from the fuel supply means adheres to the wall surfaces of the intake pipe and forms the films. These fuel films flow along the intake manifold wall into the engine cylinders in varying quantities so that the air-fuel ratios of the combustion mixtures charged into the engine cylinders vary from one cylinder to another. The adhesion of the fuel to the wall surfaces and the resultant formations of the fuel films are in turn attributed to the fact that the fuel has not been satisfactorily atomized and uniformly mixed with the combustion air.
Another cause is attributed to the butterfly type throttle valve which is not positioned symmetrically with respect to the axis of the air-fuel passage so that non-uniform distributions or the localized concentrations of the air-fuel mixtures result. In addition, the butterfly valve is so designed and arranged as to rotate, thereby varying the opening area so that the geometrical relationship between the butterfly type throttle valve and the fuel supply means changes from time to time depending upon the operating conditions of the engine and consequently the localized concentrations of fuel are further enhanced.
In the intake systems wherein the fuel injection valve is positioned at the upstream of the throttle valve, the fuel injection pressure is considerably higher than the pressures around the throttle valve and the fuel is injected independently of the flowing conditions of the air. As a result, the air-fuel ratio becomes high when the air flow rate is high, and vice versa. Thus the fuel-air ratios of the combustion mixtures charged into the engine cylinders vary from one cylinder to another and from time to time.